The present invention relates to optical branching equipment which is employed in an optical network using an optical fiber cable, for switching transmission lines. The invention also pertains to such an optical network.
A bus type optical network calls for optical branching equipment which includes an optical repeater for amplifying an attenuated optical signal and a branching equipment for switching optical signal transmission lines.
FIG. 1 illustrates in block form a prior art example of optical branching equipment.
In FIG. 1 optical fiber cables 61.sub.1 and 61.sub.3 which form a trunk line is made up of optical fibers 52.sub.1 and 52.sub.4 and feed lines 59.sub.1 and 59.sub.2. Optical branching equipment 60 is provided with optical/electrical converters 53 and 55 for converting optical signals from the optical fibers 52.sub.1 and 52.sub.3 into electrical signals, electrical/optical converters 54 and 56 for converting electrical signals into optical signals for input into optical fibers 52.sub.2 and 52.sub.4, an electric circuit 58 which switches between branching circuits for connecting the optical fibers 52.sub.2 and 52.sub.3 to the optical fibers 51.sub.1 and 52.sub.4, respectively, and a short circuit for connecting the optical fibers 52.sub.1 and 54.sub.4 to each other and which performs amplification and other signal processing, and a power circuit 57 for supplying power from the feed line 59.sub.1 to each circuit.
The optical branching equipment 60 is usually employed as optical signal branching means in an optical network which forms a bus type optical transmission line by connecting a branch line to a trunk line. That is, the electric circuit 58 normally connects the trunk line optical fiber cables 61.sub.1 and 61.sub.3 to a branch line optical fiber cable 61.sub.2 and disconnects the short circuit in terms of a signal. The branch line optical fiber cable 61.sub.2 has connected thereto branch line terminal equipment 62, which converts an optical signal from the trunk line optical fiber cable 61.sub.1 into an electrical signal by an optical/electrical converter 63, adds and/or drops required information by a signal add/drop circuit 64 and converts the electrical signal into an optical signal by an electrical/optical converter 65, thereafter sending the optical signal to the trunk line optical fiber 61.sub.3.
Upon occurrence of breakage or some other fault of the optical fiber cable 61.sub.2 which forms a branch line, the electric circuit 58 switches the transmission line to the short circuit which interconnects the optical fibers 52.sub.1 and 52.sub.4, thus disconnecting the optical fiber cable 61.sub.2.
For example, in a submarine repeatered transmission network employing submarine optical fiber cables, optical branching equipment for use in a bus type or star type optical network is required to be highly reliable for its long-term, maintenance-free operation. In the conventional optical branching equipment, however, each circuit employs many parts including semiconductor devices, which must be made highly reliable to meet the reliability requirement for the optical branching equipment--this inevitably raises the manufacturing costs of optical branching equipment.
Moreover, since the transmission line is supervised from a shore terminal, it is necessary to provide an independent transmission line for sending transmission line switching control information.